Dredged marine soils (DMS) have poor engineering properties, which limit their usage in construction projects. This research examines the application of reactive magnesia (rMgO) containing supplementary cementitious materials (SCMs) to stabilize DMS under ambient and carbon dioxide (CO₂) curing conditions. Several proprietary experimental tests were conducted to investigate the stabilized DMS. Furthermore, the carbonation-induced mineralogical, thermal, and microstructural properties change of the samples were explored. The findings show that the compressive strength of the stabilized DMS fulfilled the 7-d requirement (0.7–2.1 MPa) for pavement and building foundations. Replacing rMgO with SCMs such as biochar or ground granulated blast-furnace slag (GGBS) altered the engineering properties and particle packing of the stabilized soils, thus influencing their performances. Biochar increased the porosity of the samples, facilitating higher CO₂ uptake and improved ductility, while GGBS decreased porosity and increased the dry density of the samples, resulting in higher strength. The addition of SCMs also enhanced the water retention capacity and modified the pH of the samples. Microstructural analysis revealed that the hydrated magnesium carbonates precipitated in the carbonated samples provided better cementation effects than brucite formed during rMgO hydration. Moreover, incorporating SCMs reduced the overall global warming potential and energy demand of the rMgO-based systems. The biochar mixes demonstrated lower toxicity and energy consumption. Ultimately, the rMgO and biochar blend can serve as an environmentally friendly additive for soft soil stabilization and permanent fixation of significant amounts of CO₂ in soils through mineral carbonation, potentially reducing environmental pollution while meeting urbanization needs.